1. Field of the Invention
The present invention is related to a magnetic memory element, and in particular to a magnetic memory element utilizing spin transfer switching.
2. Description of Related Art
The magnetic random access memory (MRAM) has the advantages such as non-volatility, high density, high reading and writing speeds, and radiation resistance. When the conventional magnetic memory writes data, generally a magnetic memory cell selected by the intersection of induction magnetic fields of a write bit line (BL) and a write word line (WWL) is used to change a magnetoresistance (MR) of the magnetic memory by changing a magnetized vector direction of the free layer. When reading a memory data, a reading current flows into the selected magnetic memory cell. Afterwards, a detected resistance of the magnetic memory cell is used to determine a digital value of the memory data.
A conventional magnetic memory cell is a stacked structure formed by stacking a pinning layer of an anti-ferromagnetic material, a pinned layer of ferromagnetic/non-magnetic metal/ferromagnetic layers, a tunneling barrier layer and a free layer of a magnetic material. Through the high or low magnetoresistance (MR) derived from magnetized directions of the pinned layer and the free layer being in parallel or anti-parallel, data of a logic state “0” or a logic state “1” are recorded.
As the CMOS technique keeps influencing advanced technology and in order to respond to high-density MRAM designs, a magnetic tunnel junction (MTJ) element of the MRAM also continues shrinking in its size. For the conventional asteroid mode and the toggle mode MTJ elements, when their sizes reduce, a switching field of the free layer in the MTJ elements continues to rise, and the operating margin reduces or even disappears. Therefore, how to enhance write selectivity and reduce write currents have always been the most notable obstacles developing magnetic memories has ever encountered.
The spin torque transfer random access memory (STT-RAM) is considered as the magnetic memory most likely to be applied in technology nodes beyond 65 nm. For the STT-RAM utilizing spin transfer switching as its write mode, since write currents only pass through those selected memory elements and magnetized switching depends upon write current density, shrinkage of elements is advantageous for reduction of write currents. In theory, enhancing write selectivity and reducing write currents can be achieved simultaneously.
A theoretical estimated value of a switching critical current density of the magnetic memory element utilizing spin transfer switching is Jc=αeMst[Hk+2πMs]/hη. Ms is a saturation magnetization of a magnetic layer per unit volume; t is a thickness of the magnetic layer; Hk is an anisotropic field; α is the Gilbert damping constant; η is a spin polarization factor, and h is the Boltzmann constant. The estimated value has not considered the effects of external electric fields and thermal dithering. It is known from the above formula that in research on the spin transmission effect, the factors influencing the switching current mainly are the foregoing physical quantities (Ms, Hk, α and η). Among them, the saturation magnetization (Ms) influences most significantly.
In U.S. Publication No. 2007/0159734 A1, an STT-RAM structure is proposed, which utilizes non-magnetic materials to perform doping in the free layer, e.g., Cr, Cu, Au, B, Nb, Mo, Ta, Pt, Pd, Rh, Ru, Ag, TaN, CuN and TaCuN, thereby reducing the saturation magnetization (Ms) of the free layer so as to lower the critical current density (Jc).
However, in order to continue enhancing the performance of the STT-RAM, how to reduce the critical current density and maintain sufficient thermal stability in the STT-RAM are still urgent issues to be resolved.